A:it marked
B:marking
C:marked
D:to mark
设n∈-1,12,1,2,3,则使得f(x)=xn为奇函数,且在区间(0,+∞)上单调递减的n的个数是( )
— I can’t do it______ anymore. I need help.
北回归线以北地区,在昼长夜短期间,白昼一天比一天短的时期是( )